1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
Wireless communication systems typically include a plurality of base stations or access points that provide wireless connectivity to mobile units within a geographical area that is usually referred to as a cell. The air interface between the base station or access point and the mobile unit supports one or more downlink (or forward link) channels from the base station to the mobile unit and one or more uplink (or reverse link) channels from the mobile units to the base station. The uplink and/or downlink channels include traffic channels, signaling channels, broadcast channels, paging channels, pilot channels, and the like. The channels can be defined according to various protocols including time division multiple access (TDMA), frequency division multiple access (FDMA), code division multiple access (CDMA), orthogonal frequency division multiple access (OFDMA), as well as combinations of these techniques. The geographical extent of each cell may be time variable and may be determined by the transmission powers used by the base stations, access point, and/or mobile units, as well as by environmental conditions, physical obstructions, and the like.
Mobile units are assigned to base stations or access points based upon properties of the channels of supported by the corresponding air interface. For example, in a traditional cellular system, each mobile unit is assigned to a cell on the basis of criteria such as the uplink and/or downlink signal strength. The mobile unit then communicates with that serving cell over the appropriate uplink and/or downlink channels. Signals transmitted between the mobile unit and the serving cell may interfere with communications between other mobile units and/or other cells. For example, mobile units and/or base stations create intercell interference for all other sites that use the same time/frequency resources. The increasing demand for wireless communication resources has pushed service providers towards implementing universal resource reuse, which increases the likelihood of intercell interference. In fact, the performance of modern systems is primarily limited by intercell interference, which dominates the underlying thermal noise.
Although conventional wireless communication systems attempt to reduce the effects of intercell interference using various interference cancellation techniques, alternative approaches recognize that intercell “interference” is actually caused by signals that include valuable information. For example, on the uplink, intercell interference at one cell site is merely a superposition of signals that were intended for other cell sites, i.e., the intercell interference is formed of mobile unit signals that have been collected at the wrong place. If these signals could be properly classified and routed, they would cease to be interference and would become useful in the detection of the information they bear. While challenging, combining information received at disparate sites is theoretically possible because the cell sites are connected to a common and powerful backbone network. This is tantamount to recognizing that a network of wireless cell sites can form a large distributed multi-access channel and all users can be served through all the cell sites. This ambitious approach, which has been termed “Network Multiple-In-Multiple-Out” or “Network MIMO”, leverages the almost unlimited bandwidth available in wireline networks to transcend intercell interference and alleviate the wireless bottleneck.
A complete implementation of Network MIMO requires that all the cell sites in a system are tightly coordinated both in reception (uplink) and in transmission (downlink). However, wireless signals become progressively weaker as they propagate outward from the cell site. Consequently, coordination may be restricted to clusters of cell sites with minimal performance degradation of the Network MIMO system. Reducing the coordination from the entire system to predetermined clusters of cell sites can substantially reduce the backbone traffic required to coordinate operation of the cells.
FIG. 1 conceptually illustrates a static cell cluster pattern for a conventional network MIMO system 100. The system 100 includes a plurality of cells 105 that are grouped into clusters 110. In the interest of clarity only a single cell 105 and a single cluster 110 are explicitly referred to using the identifying numerals. Although not explicitly shown in FIG. 1, persons of ordinary skill in the art will appreciate that each cell 105 includes one or more base stations and/or access points for providing wireless coverage to the cell 105. Accordingly, the term “cell” may be used herein to refer to the geographic area and/or the corresponding base station. The clusters 110 shown in FIG. 1 are disjoint and each of them contains 3 cells. For example, one cluster 110 includes the cells 1, 3, 5 and an adjacent cluster includes the cells 2, 6, 14. The membership of cells 105 and the various clusters 110 is predetermined based upon factors such as the geography of the region including the cells 105 and/or the clusters 110. Mobile units may then be assigned to clusters 110 on the basis of factors such as the received signal powers and/or a total interference at the cells 105 in each cluster 110.
The decay in the signal strength that makes clustering feasible is, however, randomized by fading, which may vary over time and frequency. As a result, signals traveling the same distance in different parts of a given system may encounter differences of several tens of dB in attenuation because of distinct fading. Signals traveling through the same part of the system at different times may also experience different fading. For example, signals traveling from a given point in distinct directions may encounter differences of several tens of dB in attenuation. As a result, the signal strength received by cell sites (over the uplink) and/or mobile units (over the downlink) is a variable function of the environment. However, the predetermined cluster definitions cannot account for the variability of channel conditions associated with each mobile unit and/or cell site.